This invention relates to a novel block copolymer composition for use in adhesives. More particularly, it relates to branched three-armed styrene-isoprene block copolymer compositions comprised of polymeric blocks and adhesives made using such compositions.
It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced which comprise primarily those having a general structure EQU A--B and A--B--A
wherein the polymer blocks A comprise thermoplastic polymer blocks of alkenyl arenes such as polystyrene, while block B is a polymer block of a conjugated diene such as polyisoprene. The proportion of the thermoplastic blocks to the elastomeric polymer block and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics. When the content of the alkenyl arene is small, i.e. 5-30% the produced block copolymer is a so-called thermoplastic rubber. In such a rubber, the blocks A are thermodynamically incompatible with the blocks B resulting in a rubber consisting of two phases--a continuous elastomeric phase (blocks B) and a basically discontinuous hard, glass-like plastic phase (blocks A)--called domains. Since the A--B--A block copolymers have two A blocks separated by a B block, domain formation results in effectively locking the B blocks and their inherent entanglements in place by the A blocks and forming a network structure at normal use temperature.
These alkenyl arene domains act as physical crosslinks anchoring the ends of many block copolymer chains. Such a phenomenon allows the A--B--A rubber to behave like a conventionally vulcanized rubber in the unvulcanized state and is applicable for various uses. For example, these network forming polymers are applicable for uses such as in adhesive formulations; in molded shoe soles; as an impact modifier for polystyrene resins and engineering thermoplastics; for modification of asphalt; etc.
Styrenic block copolymers are widely used to make pressure sensitive adhesives for a wide variety of uses including tapes, labels and product assembly applications. Pressure sensitive adhesives based on styrenic block copolymers such as KRATON.RTM. thermoplastic rubber provide excellent adhesion to a wide variety of substrates, including paper, cardboard and other cellulosic materials. For instance, U.S. Pat. Nos. 3,239,478 and 3,935,338, which are herein incorporated by reference, disclose how to make pressure sensitive adhesives of this type. They include examples of linear S--I--S polymers, the latter at col.5 describes a polymer with a polystyrene content of 14% and a molecular weight of 145,000. High holding power in a shear test is important in such adhesives because this is a function of the load bearing capability of the adhesive, an important feature in packaging applications, for example. Thus, it would be advantageous to have available an adhesive with higher shear holding power than is presently available and also processable as a thermoplastic (melt viscosity less than 500 Pa-s at 177.degree. C.).